In imaging system design, such as for a projection system, it is necessary to correct for aberrations to improve image quality while maximizing the light throughput of the system to increase brightness. Conventionally, in a chromatic coaxial optical system, i.e., where all wavelengths of light are transmitted along a common optical system, the imaging optics are designed and optimized based on common optical elements for all wavelengths. In other words, conventional optical systems are designed with all wavelengths of light passing through the same lenses and aperture stops.
Difficulty arises in the design and optimization of optical systems because different wavelengths of light have different optical path lengths through the optical system based on their wavelength. Thus, an optical system transfer function could be different for any specific wavelength causing the system modulation transfer functions (MTF) to be wavelength dependent as well. The optimization of the MTF for one color in an optical system generally results in the degradation of the MTF for other colors, same as the optimization of one field point generally results in the degradation of the MTF for the other field points. Accordingly, compromise is generally necessary in the design and optimization of an imaging system, among different wavelengths as well as different field points.
The design of optical systems is further complicated where independent color light sources, such as light emitting diodes (LEDs) are used. The throughput of an optical system is defined by the extent of the aperture stop without any vignetting, i.e., no ray in the optical system is cut off along the optical train. Because some color light sources may be brighter than others, and because the throughput of the system is conventionally designed to be equal for all wavelengths, weaker light sources stay relatively weak at the output of the optical system. Accordingly, the optical design in imaging systems using multiple light sources is typically limited to a choice of optical elements and transmitting apertures that will provide adequate throughput for all wavelengths.